This invention relates to a television camera having multiple clocked solid-state imagers for producing a plurality of signal components which are multiplexed to a single analog-to-digital converter which is common to the signal components.
Portable color television cameras are in common use for Electronic Newsgathering (ENG) applications and are becoming commonplace for home use. The cameras currently use analog video signal processing. The synchronizing circuits are currently almost universally composed of binary circuits using signal levels representing ONE and ZERO (i.e., ON, OFF) which are sometimes referred to as digital circuits. It is known to process television video signals by the use of digital circuits representing in combination a large number of possible amplitudes, rather than only two as in the aforementioned synchronizing circuits. For example, different combinations of eight on-off digital signals can represent up to 2.sup.8 possible amplitudes, thereby representing the video signal by means of stable digital circuits. In addition to being stable, certain types of signal processing can be readily implemented in digital form which are difficult or impossible to implement in analog form, as for example circuits requiring long signal delays without change of signal amplitude.
The first step in generating a video signal by a camera involves the use of imagers for transducing light from a scene to a signal. Modern television cameras use color-splitting prisms and a plurality of solid-state imagers to produce time-quantized analog video signals representing red, green and blue components of the scene being televised. U.S. Pat. No. 4,167,755 (Nagumo) describes a camera with three CCD solid-state image sensors to produce red, blue and green-representative signals under the control of a clock or pulse generator. The video signals produced by the imagers are processed as known by low-noise amplifying or by double-correlated sample-and-hold circuits under the control of the clock generator.
If digital signal processing is desired, the analog-to-digital conversion is performed at a point in the circuit at which the analog signal is amplified to a suitable level. Analog-to-digital converters (ADC's) generally have problems of linearity, range, and offsets.
In order to avoid aliasing, the time-quantization or sampling associated with analog-to-digital conversion must be performed at a rate which is at least twice the highest frequency at the input signal which is to be reproduced. Practical considerations such as finite filter cutoff rates make it desirable to operate the ADC at rates three, four or more times higher than the highest frequency of the input signal. When the input signal is a composite color television signal, in which the color information is modulated onto a subcarrier which is added to the luminance signal at a high frequency (3.58 MHz for NTSC), the ADC may be operated at a rate of 14.3 MHz. At such rates, ADC's have additional problems of high power consumption and concomitant heat generation during operation.
It is known to use a single analog-to-digital converter to perform analog-to-digital conversion for a plurality of channels carrying components of a color television system, as described in U.S. Pat. Nos. 4,150,397 (Russell); 4,163,248 (Heitmann); 4,240,103 (Poetsch); and 4,364,080 (Vidovic). In Russell and in Heitmann, a composite color T.V. signal is decoded into components (In Poetsch, and imagers of a camera generate the components) and the components are time-division multiplexed to an ADC, and further processed. Vidovic describes a digital video analyzer which receives video from an external source, processes it to form video components, and time-division multiplexes the video components to an ADC for further processing. As alluded to by Heitmann, the time-division multiplexing results in information loss. This loss results from the fact that during the interval in which the multiplexer couples the ADC to a particular signal component, the signal components which arrive concurrently with the one being converted are lost. The loss of information necessarily results in loss of resolution.
In order to avoid the loss of resolution due to the inability of a multiplexed ADC to convert signals of channels to which it is not connected, one could couple an ADC to receive a single signal representing the totality of the luminance and chrominance information of the scene as described in U.S. Pat. No. 4,422,094 issued Dec. 20, 1983 to Lewis, Jr., et al. This may be accomplished in the context of a camera, for example, by using a single imager in known fashion with a color-stripe or checkerboard filter to produce a signal which from pixel to pixel alternates the color represented. However, such single-imager cameras have not been successful for high-quality use because of colorimetry problems, and furthermore the resolution of the image produced by such single-imager color cameras tends to be low because of the large distance between the pixels representing a particular color. For example, if the horizontal color pattern is RGBRGB, each green-representative pixel (which is the principal component of luminance) is separated by two intervening pixels.
In order to avoid loss of resolution, three imagers may be used and each color signal channel may be provided with an ADC, as described in U.S. Pat. No. 3,617,626 (Bluth). It might be thought that the required frequency of operation of each ADC could be reduced when one ADC is used for each channel rather than using one ADC for a composite signal, thereby reducing the power-consumption of each. However, the imagers of a color camera normally produce R, G and B-representative signals, which unlike the I (in-phase) and Q (quadrature) color signals are wideband signals (since all three are required to form the wideband luminance signal). Thus, use of three imagers, one for each channel, does not significantly affect the required frequency of operation. Furthermore, the aforementioned nonlinearity and offsets of the ADC's can give rise to colorimetry problems when separate ADC's are used for each channel of a camera. For example, if equal analog input signals representing R, G and B are applied to three ADC's (one for each processing channel), the nonlinearities may result in the generation of unequal R, G and B-representative digital signals. These unequal digital signals will cause colorimetry problems.
The present invention is founded upon the recognition that in the context of a portable television camera the use of digital signal processing is especially advantageous because of the relative stability of digital signal processing under the environmental conditions of cold, heat and shock to which portable cameras are subject. Further, because portable cameras are battery-powered, low power drain is desirable, so a single ADC is desirable even though there may be more than one signal transducer. In a fixed studio camera, digital video signal processing is also advantageous in that it reduces the need for alignment, and use of a single ADC provides a low parts count which is desirable for reliability and cost reasons. The loss of resolution occasioned in the aforementioned prior art by the single ADC as used therein is unacceptable for quality television.
Further, the present inventor recognized that while analog-to-digital converters tend to be limited in speed, solid-state imagers such as charge-transfer imagers are even more limited in speed, and are further limited in resolution. Thus, a camera with multiple CCD imagers, each driving a separate analog-to-digital converter, does not take advantage of the potential speed of each of the analog-to-digital converters, and furthermore has resolution limited by the finite number of light-sensing areas on the imager. Also, the present inventor recognized that solid-state charge-transfer imagers, such as CCD imagers, inherently include a clock-controllable storage function which may be advantageously used with a simple timing control to provide a time delay which simplifies time-division multiplexing.